Inkjet printing mechanisms may be used in a variety of different products, such as plotters, facsimile machines and inkjet printers, to print images using a colorant, referred to generally herein as "ink." These inkjet printing mechanisms use inkjet cartridges, often called "pens," to shoot drops of ink onto a page or sheet of print media. Some inkjet print mechanisms carry an ink cartridge with a full supply of ink back and forth across the sheet. Other inkjet print mechanisms, known as "off-axis" systems, propel only a small ink supply with the printhead carriage across the print zone, and store the main ink supply in a stationary reservoir, which is located "off-axis" from the path of printhead travel. Typically, a flexible conduit is used to convey the ink from the off-axis main reservoir to the printhead cartridge. In multi-color cartridges, several printheads and reservoirs are combined into a single unit, with each reservoir/printhead combination for a given color also being referred to herein as a "pen."
Each pen has a printhead formed with very small nozzles through which the ink drops are fired. The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
To clean and protect the printhead, typically a "service station" mechanism is mounted within the plotter chassis so the printhead can be moved over the station for maintenance. For storage, or during non-printing periods, the service stations usually include a capping system which hermetically seals the printhead nozzles from contaminants and drying. Some caps are also designed to facilitate priming, such as by being connected to a pumping unit or other mechanism that draws a vacuum on the printhead. During operation, clogs in the printhead are periodically cleared by firing a number of drops of ink through each of the nozzles in a process known as "spitting," with the waste ink being collected in a "spittoon" reservoir portion of the service station. After spitting, uncapping, or occasionally during printing, most service stations have an elastomeric wiper that wipes the printhead surface to remove ink residue, as well as any paper dust or other debris that has collected on the face of the printhead.
To print an image, the printhead is scanned back and forth across a printzone above the sheet, with the pen shooting drops of ink as it moves. By selectively energizing the resistors as the printhead moves across the sheet, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in one or more linear arrays. If more than one, the two linear arrays are located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction. Thus, the length of the nozzle arrays defines a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath width" of the pen, the maximum pattern of ink which can be laid down in a single pass.
It is apparent that the speed of printing a sheet can be increased if the swath width is increased. That is, a printhead with a wider swath would require fewer passes across the sheet to print the entire image, and fewer passes would increase the throughput of the printing mechanism. "Throughput," also known as the pages-per-minute rating, is often one of major considerations that a purchaser analyzes in deciding which printing mechanism to buy. While it may seem to the inexperienced an easy thing to accomplish, merely lengthening the nozzle array to increase throughput, this has not been the case. For thermal inkjet pens in particular, there are some physical and/or manufacturing constraints to the size of the substrate layer within the printhead. In the past, inkjet printheads have been limited in swath width to around 5.4 mm (millimeters) for tri-chamber color printheads, and around 12.5 mm (about one-half inch) for monochrome printheads, such as black printheads.
Recent breakthroughs in technology have given hope to developing a printhead with a 25 mm swath width (about one inch wide), which is double the width previously obtainable, and future developments may bring about even wider swath printheads. Unfortunately, the possibility of a wider swath width brings on other problems which have not previously been encountered, such as how to provide a uniformly level printing surface under the wider printhead. This media support issue is a significant problem in large format inkjet plotters, which feed media (e.g. paper) from a large roll for printing D-sized or E-sized engineering or architectural drawings, or posters, for instance. The length of the printzone in these plotters is often over a meter (around four feet).
In the past, with a 12.5 mm (one-half inch) wide print swath, the media was adequately supported by a roller which ran across the entire length of the printzone. Using a roller with a diameter of about 75 mm (about three inches) supported the media nearly linearly at the one-half inch print swath across the entire printzone. That is, any variation in the media-to-printhead spacing along the length of the nozzle array yielded visually acceptable deviations in print quality using the earlier smaller swath printheads. While a simple answer may be to increase the roller diameter to accommodate the new larger printhead, in a commercially viable plotter, such a larger diameter roller would not be acceptable. A larger diameter roller would not only increase the cost and weight of the plotter, but it would also increase the overall size of the plotter, an undesirable side effect in today's compact office environments.
Another significant problem in these large format plotters is advancing the media very accurately from one print swath to the next. One system for moving the media through the plotter printzone is shown in U.S. Pat. No. 5,342,133 ("the '133 patent"), which is assigned to Hewlett-Packard Company, the assignee of the present invention. The plotter of the '133 patent grips the edges of the media to move it through the printzone. While the '133 patent worked well for the earlier inkjet pens having only a 12.5 mm (one-half inch) print swath, it was unable to maintain the uniform printhead to media spacing required for a 25 mm (one inch) wide printhead because the majority of the print swath was unsupported.
In large format plotters, the stiffness of the typical media (paper) is insufficient to enable driving the media along its edges while maintaining good positional accuracy in the middle of the sheet. That is, when the media is only supported along its edges, it tends to sag under its own weight, increasing the printhead-to-media spacing near the middle of the sheet, which can blur the center of the printed image. This deficiency of the '133 patent plotter becomes even more evident when printing an image that requires a large quantity of ink, which causes the media, especially paper, to become saturated and soggy.
For instance, plotters are typically used to print engineering and architectural drawings, but recent advances in technology make the printing of enlarged photographic images (e.g. posters) on D-sized and E-sized drawings now possible, both technologically and economically. These posters typically carry images that require far more ink than the typical engineering or architectural drawing, so there is a greater tendency for a poster image to saturate the media with ink, causing an undesirable effect known in the art as "cockle." The term cockle refers to the tendency of media, such as paper, to uncontrollably bend or buckle as the wet ink saturates the fibers of the medium and causes them to expand. This buckling or cockling causes the media to uncontrollably bend either downwardly away from the printhead, or upwardly toward the printhead, with either motion undesirably changing the printhead-to-media spacing and leading to poor print quality. Moreover, upward buckling can be extreme enough to cause the media to contact the printhead and possibly clog a nozzle and/or smear ink on the media, damaging the image.
Thus, a whole new market is now open for plotters having a high print quality and a high throughput, so that high quality poster-sized images may be rapidly printed, as well as the conventional engineering and architectural drawings.